This invention concerns a process for the hydrolysis of urea contained in process water from the synthesis of urea via the reaction of ammonia and carbon dioxide, and containing amounts of urea varying between 100 and 30,000 ppm and for the possible contemporaneous elimination of residual NH.sub.3 and CO.sub.2.
This invention also includes equipment for carrying out said process, comprising at least a container enclosed in a shell, an inlet for the water containing residual urea, an inlet for the steam, an inlet for the stripping gas and/or modify the solution pH.
The synthesis of urea is based on reaction of ammonia and of CO2 in order to produce ammonium carbamate, which through high temperature dehydration produces urea according to the following exothermic reaction scheme: EQU 2NH.sub.3 +CO.sub.2 .fwdarw.(NH.sub.3).sub.2 CO.sub.2 .fwdarw.(NH.sub.2).sup.2 CO+H.sub.2 O.
The reaction produces one H.sub.2 O mole for each mole of urea, equal to 300 kilos of water for a ton of synthesized urea.
Besides this synthetic water, the plant discharges all the water entering as steam for the final concentration of urea in vacuum groups; it can thus be estimated that the amount of water evacuated from a urea plant is at 470-550 kg/t of urea produced.
The water evacuated from a urea plant contains not only urea but also ammonia and CO.sub.2 already present before hydrolysis of the urea, which takes place according to the following endothermic reaction scheme: EQU NH.sub.2 -CO-NH.sub.2 +H.sub.2 O.fwdarw.2 NH.sub.3 +CO.sub.2.
Hydrolysis of the urea must therefore be carried out with a process and in the kind of equipment capable of continuously separating the hydrolysis products (NH.sub.3 and CO.sub.2), to move the reaction towards the right.
The water contaminated by NH.sub.3, CO.sub.2, and urea coming from the urea synthesis plant must be treated beforehand according to known technique to separate the existing ammonia and CO.sub.2 so as to assist the hydrolysis reaction, and the hydrolysis equipment must be planned in such a way as to separate by stripping the products of decomposition as they are produced according to the reaction schematized above.
The ammonia and the CO.sub.2, being the components for the synthesis of urea, are recovered from the hydrolysis reaction through known technique.
Current provisions for the protection of the environment forbid the discharge of water coming from urea plants to avoid the grave problem of eutrophy.